1. Field of the Invention
The invention relates to a zoom system for an illumination device, in particular, an illumination device of a microlithographic projection exposure system.
2. Description of the Related Art
One purpose of illumination devices employed on microlithographic projection exposure systems is to uniformly illuminate a reticle arranged in the object plane of a projection lens that follows the reticle in the optical train in a manner that has been adapted to accurately suit the optical properties of the projection lens. The illumination should be telecentric in order that the directed foci of all points in the plane of the reticle will be as accurately centered on the projection lens' entrance pupil as possible. In addition, it may be desirable to provide a partially coherent illumination for which the extent to which that entrance pupil is filled will be variable and adjustable. Zoom systems are usually employed for varying the degree of coherence of the illumination. In order to allow reaching a close approximation to the limits of resolution of the optical projection during the photolithographic micropatterning process, the illumination is frequently optimized to suit the patterns on the individual layouts by creating various illumination modes, for example, annular illumination or quadrupole illumination. Devices, such as conical or pyramidal axicons, for that purpose may be incorporated into zoom systems, since there is a demand for high illumination efficiency allowing utilizing the light outputs of the light sources employed for fabricating microdevices with the least possible light losses.
Illumination devices that meet that demand are disclosed in, for example, European Patent EP 0 747 772, German Patent DE 44 21 053, or European Patent EP 0 687 956. In the case of the illumination system of European Patent EP 0 747 772, the zoom system has a plurality of lenses that are arranged along an optical axis and define an object plane and an image plane that is a Fourier transform of the object plane. Two of its lenses are movable lenses that are movable or repositionable along the optical axis when setting zooming positions of the zoom system in order to vary the size of an illuminated area in the image plane. Graticular, diffractive, optical elements bearing two-dimensional, graticular patterns are arranged in both the object plane and the exit pupil of the zoom lens. This arrangement serves to increase the light guidance factor in a suitable manner, where that graticular optical element that is arranged in the object plane, together with the zoom system, introduces a small portion of the light guidance factor and that graticular optical element that is arranged in the image plane generates the major share of the light guidance factor and adapts the illumination to suit the size and shape of the illuminated field, for example, the rectangular entrance aperture of a rod-shaped light integrator that follows it in the optical train. The graticular elements may also be called raster elements or rastered elements. The zoom system has a zoom ratio (expansion factor) of three, where partially coherent illumination having degrees of coherence ranging from 0.3 to 0.9 may be set.
A zoom system for an illumination device of a microlithographic projection system, where the zoom system has a zoom ratio of four and allows setting degrees of coherence ranging from 0.1 to 0.4 is known from U.S. Pat. No. 5,379,090.
Employment of a zoom system on the illumination device of a wafer steppers in order to allow adjusting the degree of coherence of the illumination with low light losses is known from U.S. Pat. No. 5,237,367.
An afocal zoom system for providing illumination on wafer steppers that also allows adjusting the degree of coherence of the illumination with low light losses is known from U.S. Pat. No. 5,245,384.
An illumination system for a microlithographic projection system that allows selecting various illumination settings without employing a zoom system is known from European Patent EP 1 109 067, which corresponds to U.S. patent application 2001 001247 A1. The intensity distribution of the light beam at a pupillary plane of the illumination system is determined by a train of optical elements, at least one of which is interchangeable with another optical element using an interchanging device. These interchangeable optical elements may be formed from diffractive optical elements (DOE), microlens arrays, or holographic optical elements that individually define differing illumination settings. The devices required for interchanging those optical elements must be rapid-acting and operate with high precisions, which means that extremely elaborate mechanisms and control systems are required for implementing this sort of concept.
In many applications, in particular, applications in the field of microlithographic fabrication of semiconductor devices and other types of microdevices, it is desirable to be able to switch between various illumination settings without having to interchange any optical elements. In addition, it is frequently desirable to be able to select widely differing conventional illumination settings. A demand that is becoming increasingly important, particularly at short wavelengths, for example, 193 nm, 157 nm, or shorter wavelengths, is reducing the total number of optical surfaces in the system to a minimum in order to keep transmission losses within reasonable bounds. Furthermore, a telecentricity of the exit end (image end) of the zoom system may be beneficial in order to allow adapting it to suit the optical systems that follow it in the optical train, particularly in the case of illumination systems on which exclusively angle-maintaining optical elements, such as rod-shaped light integrators, are arranged following their zoom system.